The subsidiary of Atlanta-based Southern Co. says it still can't reliably run parts of the plant, including units that convert lignite coal into gas.
Southern said in a stock filing Monday that it's pushing back completion by another month to May 31, more than three years behind schedule and more than $4 billion over budget.
Mississippi Power says it's absorbing $38 million in additional costs announced Monday, while customers could be asked to pay $20 million.
Overall, customers could be asked to pay $4.3 billion. Southern shareholders have lost $2.9 billion.
Mississippi Power must submit a Kemper financing plan to the state Public Service Commission by June 3. That body decides how much customers pay.